Rear evaporator core freeze protection method

ABSTRACT

One embodiment relates to a method for preventing evaporator core freezing in an air conditioning system, comprising determining the occurrence of an evaporator core freezing condition for a first one of a plurality of evaporators. Determining the occurrence of an evaporator core freezing condition may be done without measuring the air out temperature first evaporator. The method also comprises adjusting an operating parameter of the air conditioning system to eliminate the core freezing condition.

BACKGROUND

The present application relates generally to the field of vehicle airconditioning systems. In particular, the application relates to a methodand system for rear evaporator freeze protection.

In some automotive applications it is desirable to include a dualevaporator air conditioning system. Such systems allow for a moreconsistent temperature to be maintained throughout the cabin than asingle evaporator system. Alternatively, these systems may be used toallow a different set temperatures for different areas of the occupantcabin.

In automotive applications, it is generally preferred to utilize a fixeddisplacement compressor. While more expensive variable compressors canbe used, they typically are not as reliable as fixed displacementcompressors. Accordingly, the control of the compressor operation isusually binary in nature, in that the compressor motor is either engagedor it is not engaged. A clutch may be used to engage or disengage thecompressor motor.

One problem that may occur with automotive air conditioning systems isfreezing of condensate water on the evaporator core. When the evaporatorcore freezes in this way, the evaporator core or other components of thesystem may be damaged. Also, the accumulation of ice on the evaporatorcore reduces cooling efficiency and constricts air flow. In dualevaporator systems both cores are susceptible to freezing and either mayfreeze independently of the other. In these systems, sensors (such as afront evaporator air out temperature sensor) may be used to detectfreeze conditions for the front evaporator. While similar sensors may beused in connection with the rear evaporator, such configurationsincrease hardware costs.

Various methods may be used to prevent rear evaporator core freezingwithout the expense of additional sensors. For example, a lowperformance evaporator may be utilized for the rear evaporator. In suchsystems, the rear evaporator is designed such that it will not reachtemperatures below freezing. One drawback of these systems is lowercooling performance of the air conditioning system. Another drawback islower efficiency that results in higher energy consumption by thesystem.

Another alternative is to increase the rear suction line pressure drop.This also has drawbacks. This technique also results in lower coolingperformance and inefficient operation.

A third alternative is to increase the minimum airflow rate over therear evaporator. However, such a configuration reduces a user's abilityto control the system. This may result in user dissatisfaction as aresult of not being able to select a lower air flow rate.

While all of these methods may be capable of preventing rear evaporatorcore freezing, they all have drawbacks that limit system performance orcontrollability. Accordingly, there is a need for an improved methodand/or system for the prevention of rear evaporator core freezing. Thereis also a need for a system that actively predicts and reacts toevaporator core freezing conditions.

SUMMARY OF THE INVENTION

One embodiment relates to a method for preventing evaporator corefreezing, comprising determining the occurrence of an evaporator corefreezing condition for a first one of a plurality of evaporators in anair conditioning system. Determining the occurrence of an evaporatorcore freezing condition may be done without measuring the air outtemperature for the first evaporator. The method also comprisesadjusting an operating parameter, of the air conditioning system, e.g.,by use of a controller, to eliminate the core freezing condition.

Other embodiments relate to a method for preventing evaporator corefreezing in a vehicle air conditioning system having a plurality ofevaporators. One method comprised detecting an operating condition for afirst evaporator. The method also comprises detecting an environmentalcondition for the vehicle interior, and determining the occurrence of anevaporator core freezing condition for a second evaporator withoutmeasuring the air out temperature for the second evaporator. The methodfurther includes adjusting an operating parameter for the secondevaporator, e.g., by use of a controller, to eliminate the core freezingcondition.

One preferred air conditioning system for a vehicle comprises a firstevaporator core, a second evaporator core, and a temperature sensor fordetecting the air out temperature for the first evaporator core. Thesystem further comprises a processor configured to determine theoccurrence of a freezing condition for the second evaporator corewithout an air out temperature measurement for the second evaporatorcore, and a controller. The controller includes controller logicconfigured to adjust an operating parameter to eliminate the freezingcondition for the second evaporator core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a dual evaporator vehicle air conditioningsystem.

FIG. 2 is a block diagram for an air conditioning control system.

FIG. 3 is a correlation between calculated air out temperature andmeasured air out temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a dual evaporator air conditioning system 10. In someembodiments, the system may be used in a vehicle such as an automobile,truck, airplane, rail car or other vehicle. The system generallyincludes a compressor 12, a condenser 14, first expansion device 16,first evaporator 18, first blower motor 20, second expansion device 22,second evaporator 24, and second blower motor 26. The components arearranged to provide a refrigeration loop with two evaporators inparallel. A vaporized refrigerant, such as R134a or other suitablerefrigerant, may be compressed by compressor 12. The compressedrefrigerant is then condensed in condenser 14 that is cooled by outsideair. A portion of the condensed refrigerant may then be routed to thefirst expansion device 16 and first evaporator 18. Air from outside thevehicle or recirculated from the cabin may be blown over firstevaporator 18 by first blower motor 20. The resulting cool air stream isdirected to the cabin e.g., passenger compartment, of the vehicle tocool a portion of the cabin.

In many applications it is advantageous to provide a second evaporatorto allow cool air to be introduced to the cabin in more than onelocation. This allows for a more uniform cabin temperature than a singleevaporator refrigerant loop could provide due to imperfect distributionof the cool air into the cabin. Alternatively, when coupled withadditional control capabilities, a dual evaporator system can be used toprovide multiple temperature zones in order to accommodate multipleoccupants who may have different desired air temperatures.

Accordingly, in the dual evaporator air conditioning system 10, aportion of the condensed refrigerant is or may be directed to the secondexpansion device 22 and the second evaporator 24. A second blower motormay be used to recirculate cabin air over second evaporator 24 toprovide a second cool air stream. The expanded refrigerant portions fromthe first evaporator 18 and the second evaporator 24 may then becombined and directed to the compressor 12 to complete the refrigerantloop. In some embodiments one or more valves (i.e., valves 28 and 30)may be used to control the refrigerant flow to the first and secondevaporators. In other embodiments, in particular some vehicleapplications, valves 28 and 30 may be omitted.

As previously stated, evaporator core freezing is a problem that shouldbe avoided when operating an air conditioning system. One way to detectevaporator freezing is to measure the air out temperature from theevaporator core. If the air stream leaving an evaporator is at orslightly above 0° C. it is very likely that water vapor is condensingand freezing on the evaporator core. The maximum air out temperaturethat corresponds to evaporator core freezing will vary by system butwill generally be above the freezing point of water as the airtemperature will increase between the evaporator core and the outlet.This air out temperature that corresponds to freezing of the evaporatorcore is the critical freezing temperature for the evaporator core(T_(CR)). The resultant ice build up can place mechanical stresses oncomponents of the evaporator and other equipment that can result indamage to the system. Also, the accumulation of ice on the evaporatorcore reduces cooling efficiency and constricts air flow.

As shown in FIG. 1, system 10 also includes a controller 32, and sensors34, 36, and 38. The sensors may be coupled directly to a controller orindirectly through a BUS or other device. Sensor 34 is positioned in thecool air stream leaving first evaporator 18 and measures the temperatureof the stream. Sensor 34 is coupled to controller 32. Controller 32 maybe configured to control a clutch 13 on compressor 12. Controller 32 maythen use a controller logic whereby controller 32 disengages thecompressor motor when sensor 34 detects that the cool air stream leavingfirst evaporator 18 is at or below 0° C.

Sensor 36 is positioned to detect the air temperature within the vehiclecabin and is coupled to controller 32. Sensor 38 is configured tomeasure the air temperature outside the vehicle and is also coupled tocontroller 32. Many vehicles include interior and exterior airtemperature sensors that may be utilized without the cost of additionalhardware.

Controller 32 is also coupled to first blower motor 20 and second blowermotor 26. Controller 32 may be configured to monitor and control thevoltage across the blower motors such that the controller can vary theblower motor speed by varying the voltage.

While the system and method are especially useful to enhance theperformance of systems using a fixed displacement compressor, it issimilarly useful to enhance the performance of a system utilizing avariable compressor.

FIG. 2 illustrates a control algorithm to detect and/or prevent freezingof a second evaporator without requiring a temperature sensor formeasuring the temperature of the cool air stream leaving the secondevaporator. Controller 32 obtains data from the sensors and motors towhich controller 32 is coupled. In some embodiments, the data includesan air out temperature for the first evaporator (detected by sensor 34),a cabin air temperature (detected by sensor 36), and an outside airtemperature (detected by sensor 38). Additionally, controller 32 maydetermine the voltage across each of first blower 20 and second blower26.

A processor may include software that allows the processor to calculatea value that correlates to the occurrence of the second evaporatorfreezing. In some of these embodiments, the processor may be programmedwith an equation to predict the air out temperature (T_(S)) of thesecond evaporator (24 in FIG. 1). The calculated value is then comparedto a predetermined value that corresponds to evaporator core freezing(i.e. the critical freezing temperature T_(CR)). In other embodiments,the processor may be configured to predict another value indicative ofevaporator core freezing. One such calculation may be made using the airout temperature for the first evaporator, the cabin air temperature, anoutside air temperature, first blower voltage, and the second blowervoltage. These values may be correlated to the air out temperature forthe second evaporator by a regression model. This allows the air outtemperature for the second evaporator to be approximated without theaddition of another temperature sensor.

EXAMPLE

Table 1 includes data that was collected and used to develop aregression model.

TABLE 1 Rear Front Rear Front Air Out Cabin Air Outside Air BlowerBlower Evaporator Temp - Temperature Temperature Voltage Voltage Air OutData [C.] [C.] [V] [V] Temp [C.] [C.] 23 45 4 4 8.3 1.5 11.5 25 12.511.5 4.4 3.5 12 25 8 8 2.06 1.5 6 15 12.5 11.5 −3.2 −2.5 7 15 8 8 −1.1−1.8 6 5 8 8 −1.1 −1.7 15 30 4.2 4.1 4.2 1.5 14 20 4.2 4.1 2.6 0.5 7 158 8 −1.35 −1.86 12 25 8 8 1.76 1.59 20 25 7.1 6.3 3 4 15 20 11.5 11.54.6 7 13.5 20 7 6.2 2.3 1.15 13 20 7 6.2 1.8 0.4 20 45 4 4 6.9 1.34 1325 11.5 12.5 4.6 3.9 6 15 11.5 12.5 −3.2 −2.5 6 5 8 8 −1.2 −0.5

It has been found that the air out temperature for the second evaporator(T_(s)) may be approximated by use of the equation:

T _(s) =X+A T _(C) +B T _(O) +C V _(F) +D V _(R) +E T _(F)  Equation 1

where

T_(C) is the cabin air temperature

T₀ is the outside air temperature

V_(F) is the first blower motor voltage

V_(S) is the second blower motor voltage

T_(F) is the first evaporator air out temperature

Values for the parameters (A, B, C, D, E, and X) can be determined byconventional linear regression methods. While the equation has aninfinite number of solutions for the parameters, typical values will bein the ranges shown in Table 2 depending on the vehicle and typicaloperating conditions. While the values and ranges shown in Table 2 aretypical, other values and ranges may be used.

TABLE 2 Range of Values for a Parameter Value Chosen Typical A/C SystemIntercept X −4.887 −10 to 10 Cabin Temperature A 0.394 −2 to 2 OutsideTemperature B −0.190 −2 to 2 Front Voltage C 0.281 −2 to 2 Rear VoltageD 0.194 −2 to 2 Front Temperature E 0.711 −2 to 2

FIG. 3 illustrates the correlation between the air out temperature forthe second evaporator (T_(S)) predicted by use of Equation 1 where A, B,C, D, E, and X are given the chosen value from Table 2. As can be seenfrom FIG. 3, the predicted value correlates sufficiently well with themeasured value.

After the air out temperature for the second evaporator is predicted,the controller may determine if the value (T_(S)) is at or below 0° C.If T_(S) is at or below 0° C., the controller will take correctiveaction. In some embodiments, the controller may increase the voltage tothe second blower motor to increase the heat transferred to the secondevaporator and thus raise the air out temperature for the secondevaporator. In other embodiments, the controller may disengage thecompressor motor to allow the second evaporator temperature to rise.Other suitable corrective actions, including decreasing the flow rate ofrefrigerant to the second evaporator (e.g. in a system includingoptional valves 28 and 30 shown in FIG. 1), or any other suitable actionmay be taken.

The controller will continually sample the data collected by the sensorsand recalculate T_(S). This may be done at any suitable sampling rate.In some embodiments, sensor readings may be sampled at a rate of aboutonce every second. When T_(S) reaches a temperature above 0° C. (oranother preset temperature), the air conditioning system will bereturned to normal operation. The controller may also include ahysteresis loop to avoid over compensation by the controller.

Although the foregoing has been described with reference to exemplaryembodiments, workers skilled in the art will recognize that changes maybe made in form and detail without departing from the spirit and scopethereof. For example, although different example embodiments may havebeen described as including one or more features providing one or morebenefits, it is contemplated that the described features may beinterchanged with one another or alternatively be combined with oneanother in the described example embodiments or in other alternativeembodiments. The present subject matter described with reference to theexample embodiments and set forth in the following claims is manifestlyintended to be as broad as possible. For example, unless specificallyotherwise noted, the claims reciting a single particular element alsoencompass a plurality of such particular elements. Many other changesand modifications may be made to the present invention without departingfrom the spirit thereof. The scope of these and other changes willbecome apparent from the appended claims. The steps of the methodsdescribed herein may be varied, and carried out in different sequences.

1. A method for preventing evaporator core freezing in an air conditioning system having a first evaporator core and a second evaporator core, the method comprising: determining an occurrence of an evaporator core freezing condition for the second evaporator cores in the air conditioning system without measuring the air out temperature for the second evaporator core; and adjusting an operating parameter of the air conditioning system to eliminate the core freezing condition.
 2. The method of claim 1, wherein determining the occurrence of an evaporator core freezing condition comprises detecting a plurality of operating conditions and calculating a predicted value corresponding to the core freezing condition.
 3. The method of claim 2, wherein detecting comprises detecting an air out temperature for the first evaporator, a vehicle interior air temperature, and an outside air temperature.
 4. The method of claim 3, further comprising determining a voltage for a first blower motor coupled to the first evaporator, and a voltage for a second blower motor coupled to the second evaporator.
 5. The method of claim 3, wherein determining the occurrence of an evaporator core freezing condition further comprises calculating a value that correlates to the presence and/or absence of the freezing condition.
 6. The method of claim 5, wherein the value is calculated from an air out temperature for the first evaporator, a vehicle interior air temperature, and an outside air temperature.
 7. The method of claim 6, wherein the calculated value is an air out temperature for the second evaporator.
 8. The method of claim 7, wherein the air out temperature for the second evaporator is calculated according to the formula: T _(S) =X+A T _(C) +B T _(O) +C V _(F) +D V _(R) +E T _(F) where: T_(C) is the vehicle interior air temperature; T₀ is the outside air temperature; V_(F) is the first blower motor voltage; V_(S) is the second blower motor voltage; T_(F) is the air out temperature for the first evaporator; and A, B, C, D, and E are all constants.
 9. A method for preventing evaporator core freezing in a vehicle air conditioning system having a plurality of evaporators, the method comprising: detecting an operating condition for a first evaporator; detecting an environmental condition for the vehicle interior; determining an occurrence of an evaporator core freezing condition for a second evaporator without measuring the air out temperature for the second evaporator; and adjusting an operating parameter for the second evaporator, to eliminate the core freezing condition.
 10. The method of claim 9, wherein detecting an operating condition for a first evaporator comprises detecting an air out temperature for the first evaporator.
 11. The method of claim 10, further comprising determining a voltage for a first blower motor coupled to the first evaporator, a voltage for a second blower motor coupled to the second evaporator, detecting a vehicle interior temperature, and detecting a temperature outside the vehicle.
 12. The method of claim 9, wherein determining the occurrence of an evaporator core freezing condition comprises calculating an air out temperature for the second evaporator.
 13. The method of claim 12, wherein calculating comprises using a linear equation to predict an air out temperature for the second evaporator from the operating condition for the first evaporator, and the environmental condition for the vehicle interior.
 14. The method of claim 12, wherein the value is calculated from the air out temperature for a first evaporator, the vehicle interior air temperature, and the outside air temperature.
 15. The method of claim 9, wherein adjusting an operating parameter for the second evaporator comprises increasing a blower motor voltage for a blower motor coupled to the second evaporator.
 16. The method of claim 9, wherein adjusting an operating parameter for the second evaporator comprises disengaging a compressor that is in fluid communication with the second evaporator.
 17. An air conditioning system for a vehicle, comprising: a first evaporator core; a second evaporator core; a temperature sensor for detecting the air out temperature for the first evaporator core; a processor configured to determine occurrence of a freezing condition for the second evaporator core without an air out temperature measurement for the second evaporator core; and a controller having controller logic configured to adjust an operating parameter of the air conditioning system to eliminate the freezing condition for the second evaporator core.
 18. The air conditioning system of claim 17, further comprising a first blower motor coupled to the first evaporator core, and a second blower motor coupled to the second evaporator core.
 19. The air conditioning system of claim 18, wherein the controller logic is configured such that the controller increases a voltage to the second blower motor when the processor determines the occurrence of a freezing condition for the second evaporator.
 20. The air conditioning system of claim 18, wherein the processor is configured to calculate the air out temperature for the second evaporator.
 21. The air conditioning system of claim 20, wherein the air out temperature for the second evaporator is calculated from the air out temperature of the first evaporator, a vehicle interior air temperature, and an outside air temperature, using a linear equation. 